As the integration density of semiconductor devices continues to increase, the distance from contact holes that connect lower and upper interconnection layers to surrounding interconnections typically decreases and the aspect ratio of the contact holes typically increases. Thus, highly integrated semiconductor devices adopting a multilayered interconnection structure may require more accurate and strict processing conditions when contact holes are formed by using photolithography techniques. In manufacturing semiconductor devices having a design rule of 0.25 .mu.m or less, current lithography techniques may not be sufficient to reproducibly perform desirable processes with the same accuracy.
In order to overcome limitations of photolithography in the formation of contact holes, self-alignment techniques have been suggested for forming contact holes. For example, self-alignment techniques employing nitride spacers as an etch stop layer in the formation of self-aligned contact holes have been suggested.
A conventional self-alignment technique is illustrated in FIG. 2. In FIG. 2A, a lower structure, for example, a conductive layer 122 such as a gate electrode having a substantially rectangular section, is initially formed on a semiconductor substrate 120 via patterning provided by a general photolithography process. In FIG. 2B, a layer of nitride 124 is deposited on the entire surface of the conductive layer 122. In FIG. 2C, the resultant structure is subjected to an etchback process so as to form nitride spacers 124a on the sidewalls of the conductive layer 122. In FIG. 2D, an interlayer dielectric (ILD) film 126 is formed of an oxide layer on the structure. In FIG. 2E, a photoresist pattern 128 is formed on the ILD film 126 for exposing contact holes. In FIGS. 2F and 2G, the ILD film 126 is etched to form self-aligned contact hole 130.
In the conventional self-aligned contact hole formation, the ILD film 126 is etched with a high degree of selectivity relative to the nitride spacers 124a, to form the contact hole 130. During the etching process, carbon rich carbon fluoride gases, for example, C.sub.4 F.sub.8 or C.sub.5 F.sub.8, may be used so as to increase the selectivity.
These gases may typically produce a large amount of polymers. Thus, if the etching conditions are set such that selectivity is increased, the amount of polymers produced by the etching increases. As a result, the etching may be stopped after formation of the ILD layer 126a but before formation of contact hole 130, as illustrated, for example, in FIG. 2F.
Alternatively, as illustrated in FIG. 2G, when the selectivity between the ILD film 126 and the nitride spacers 124a is decreased, ILD layers 126a and a complete contact hole 130 can be formed without the interruption due to the polymer. However, when the selectivity is low, the nitride spacers 124a may be etched together with the ILD film 126 during the etching process. Accordingly, the width Wn of the remaining nitride spacers 124a may be too small to provide a desired degree of insulation layer between the sidewalls of the conductive layers 122 and the contact hole 130. Thus, a short between a self-aligned contact plug in the contact hole 130 and the conductive layers 122 may occur, for example, at exposed sidewall 122s.
In the self-aligned contact hole formation for manufacturing large scale integration semiconductor devices, the process margin may be small even under optimal processing conditions, and thus it may be difficult to reproducibly produce devices with the same accuracy.